Process for producing dicarboxylic acid

ABSTRACT

A process produces a corresponding dicarboxylic acid by subjecting a cycloalkane to an oxidative cleavage reaction with oxygen in the presence of a catalyst in a liquid phase using a continuous reactor, in which a residence time τ (hr) satisfies the following condition: 0.1≦τ≦50/c, wherein c is the proportion (% by weight) of the cycloalkane to the total weight of a charged liquid. The catalyst includes, for example, cobalt compounds, manganese compounds, and mixtures of these compounds, as well as imide compounds having at least one cyclic imide skeleton.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing a dicarboxylicacid. More specifically, it relates to a process for producing adicarboxylic acid by subjecting a cycloalkane to an oxidative cleavagereaction with oxygen in the presence of a catalyst using a continuousreactor to thereby yield a corresponding dicarboxylic acid. Suchdicarboxylic acids are useful as, for example, raw materials forpolyamides and polyesters, additives for polymers, and intermediatematerials for fine chemicals. Among them, adipic acid is typicallyimportant as a raw material for nylon 66 (polyamide 66).

2. Description of the Related Art

Certain processes of oxidatively cleaving a mixture of a cycloalkanoneand a cycloalkanol are known as processes for producing dicarboxylicacids. For example, adipic acid, a raw material for polyamides, isproduced by a process of converting cyclohexane by oxidation with airinto a mixture of cyclohexanone and cyclohexanol, and oxidizing themixture with nitric acid. However, this process invites large amounts ofnitrogen oxides, which are believed to be global warming gases, duringoxidation with nitric acid and requires enormous facilities and effortsfor disposal of the nitrogen oxides.

As a possible solution to the problem, a process of directly oxidizing acycloalkane with oxygen to thereby yield a corresponding dicarboxylicacid has been investigated as a process without by-production ofnitrogen oxides. The process, if feasible, can markedly reduceproduction process steps and production cost of dicarboxylic acids.

For example, processes of oxidizing cyclohexane in one step to yieldadipic acid have been studied since 1960s (e.g., Japanese UnexaminedPatent Application Publication No. 49-100022, PCT InternationalPublication No. WO 9407834, Japanese Patents No. 3197518 and No.3056790). However, no plant has been launched in commercial production.The reasons are as follows. These processes have been conventionallystudied mainly using batch-system reactors in which if the conversionfrom cyclohexane increases for increasing productivity of dicarboxylicacid, a reaction time increases, and the increased reaction time invitesincreased by-production of glutaric acid, succinic acid, and otherdicarboxylic acids than the target adipic acid and increasedby-production of esters, lactones, high-boiling compounds, and otherby-products. Accordingly, the processes require complicated purificationprocess steps, invite a decreased utilization of cyclohexane to therebyinvite increased production cost of adipic acid. In addition, suchby-products deteriorate the catalytic activity. Further, thebatch-system reactors require excessively large cost of construction andequipment of plants and are low in operability in commercial production.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a processfor producing a corresponding dicarboxylic acid in a high space-timeyield by catalytic oxidation of a cycloalkane with oxygen.

Another object of the present invention is to provide a process forproducing a corresponding dicarboxylic acid by catalytic oxidation of acycloalkane with oxygen with minimized deterioration in catalyticactivity.

After intensive investigations to achieve the above objects, the presentinventors have found that a target dicarboxylic acid can be produced ina high space-time yield by oxidative cleavage of a cycloalkane withoxygen in the presence of a catalyst using a continuous reactor for aspecific residence time. The present invention has been accomplishedbased on these findings.

Specifically, the present invention provides a process for producing adicarboxylic acid including the step of subjecting a cycloalkane to anoxidative cleavage reaction with oxygen in the presence of a catalyst ina liquid phase using a continuous reactor to thereby yield acorresponding dicarboxylic acid, in which a residence time τ (hr)satisfies the following condition:0.1≦τ≦50/c

wherein c is the proportion (% by weight) of the cycloalkane based onthe total weight of a charged liquid.

As the catalyst, one of cobalt compounds, manganese compounds andmixtures of these compounds may be used. The catalyst may also be animide compound having at least one cyclic imide skeleton represented byfollowing Formula (I):

wherein n is 0 or 1; and X is an oxygen atom or a —OR group, wherein Ris a hydrogen atom or a hydroxyl-protecting group. Such imide compoundsinclude, for example, compounds represented by following Formula (1):

wherein n is 0 or 1;

-   -   X is an oxygen atom or a —OR group        -   wherein R is a hydrogen atom or a hydroxyl-protecting group;    -   R¹, R², R³, R⁴, R⁵ and R⁶ are the same or different and are each        one selected from the group consisting of a hydrogen atom,        halogen atoms, alkyl groups, aryl groups, cycloalkyl groups,        hydroxyl group, alkoxy groups, carboxyl group, substituted        oxycarbonyl groups, acyl groups, and acyloxy groups,    -   wherein at least two of R¹, R², R³, R⁴, R⁵ and R⁶ may be        combined to form one of a double bond, an aromatic ring and a        non-aromatic ring, and    -   wherein one or more of the N-substituted cyclic imido group        indicated in Formula (1) may be formed on at least one of R¹,        R², R³, R⁴, R⁵ and R⁶ and/or on the double bond, aromatic ring        or non-aromatic ring formed by the at least two of R¹, R², R³,        R⁴, R⁵ and R⁶.

A carboxylic acid can be used as a reaction solvent. A reactiontemperature is preferably from 80° C. to 150° C., and a reactionpressure is preferably equal to or more than 0.5 MPa.

The process of the present invention can produce a correspondingdicarboxylic acid in a high space-time yield by catalytic oxidation of acycloalkane with oxygen with minimized deterioration in catalyticactivity.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Cycloalkanes

Cycloalkanes (hereinafter briefly referred to as “substrate”) are usedas a raw material in the present invention.

Such cycloalkanes include, but are not limited to, cyclopropane,cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane,cyclononane, cyclodecane, cyclododecane, cyclotetradecane,cyclohexadecane, cyclooctadecane, cycloicosane, cyclodocosane,cyclotriacontane, and other cycloalkanes each having from about 3 toabout 30 members. Among them, cyclopentane, cyclohexane, cyclooctane,cyclododecane, and other cycloalkanes each having about 5 to about 15members are preferred, of which cyclohexane and cyclododecane aretypically preferred.

The cycloalkanes may have at least one substituent within ranges notadversely affecting the reaction. Such substituents include, but are notlimited to, halogen atoms, oxo group, hydroxyl group, mercapto group,substituted oxy groups (e.g., alkoxy groups, aryloxy groups, and acyloxygroups), substituted thio groups, carboxyl groups, substitutedoxycarbonyl groups, substituted or unsubstituted carbamoyl groups, cyanogroup, nitro group, substituted or unsubstituted amino groups, alkylgroups (e.g., methyl, ethyl, isopropyl, t-butyl, hexyl, octyl, decyl,and other C₁-C₂₀ alkyl groups, of which C₁-C₄ alkyl groups arepreferred), alkenyl groups, alkynyl groups, cycloalkyl groups,cycloalkenyl groups, aryl groups (e.g., phenyl, and naphthyl groups),aralkyl groups (e.g., benzyl group), and heterocyclic groups. Anaromatic or non-aromatic carbon ring or heterocyclic ring may becondensed with the cycloalkane ring of the cycloalkanes within rangesnot adversely affecting the reaction. The cycloalkanes can therefore bebridged hydrocarbons.

A corresponding cycloalkanol and/or a cycloalkanone may be added to areaction system in addition to the cycloalkane. These compounds can beconverted into a corresponding dicarboxylic acid.

Oxygen

As oxygen, any of molecular oxygen and nascent oxygen can be used. Suchmolecular oxygen is not specifically limited and includes pure oxygen,air, and oxygen diluted with an inert gas such as nitrogen gas, heliumgas, argon gas, and carbon dioxide gas. Oxygen can be formed in thereaction system. The amount of oxygen varies depending on the type ofthe substrate but is generally equal to or more than 0.5 mole (e.g.,equal to or more than 1 mole), preferably from about 1 to about 100moles, and more preferably from about 2 to about 50 moles per mole ofthe substrate. Excess moles of oxygen to the substrate is often used.Molecular oxygen can be supplied to a gas phase or a liquid phase in thereactor.

Catalysts

Catalysts for use in the present invention are not specifically limitedas long as they are oxidation catalysts that can convert cycloalkanesinto corresponding dicarboxylic acids. Preferred catalysts includecobalt compounds, manganese compounds, and other transition metalcompounds. The cobalt compounds include, but are not limited to, cobaltformate, cobalt acetate, cobalt propionate, cobalt naphthenate, cobaltstearate, cobalt lactate, and other organic acid salts; cobalthydroxide, cobalt oxide, cobalt chloride, cobalt bromide, cobaltnitrate, cobalt sulfate, cobalt phosphate, and other inorganiccompounds; acetylacetonatocobalt, other complexes, and other divalent ortrivalent cobalt compounds. The manganese compounds include, but are notlimited to, manganese formate, manganese acetate, manganese propionate,manganese naphthenate, manganese stearate, manganese lactate, and otherorganic acid salts; manganese hydroxide, manganese oxide, manganesechloride, manganese bromide, manganese nitrate, manganese sulfate,manganese phosphate, and other inorganic compounds;acetylacetonatomanganese, other complexes, and other divalent ortrivalent manganese compounds. Each of these transition metal compoundscan be used alone or in combination. Among them, cobalt compounds,manganese compounds, and mixtures of these compounds are preferred.

When such a transition metal compound is used as the catalyst, theamount thereof is, for example, from about 1 to about 200 mmol, andpreferably from about 5 to about 100 mmol per kg of the total chargedliquid.

Imide Compounds

Imide compounds having a cyclic imide skeleton represented by Formula(I) can also be used as the catalyst in the present invention. The imidecompound can be used in combination with transition metal compound(s)such as the cobalt compound and/or manganese compound. The combinationuse of the imide compound with the transition metal compound as thecatalyst may significantly improve the rate and/or selectivity of thereaction.

The bond between the nitrogen atom and X in Formula (I) is a single ordouble bond. The imide compound may have a plurality of theN-substituted cyclic imide skeleton represented by Formula (I). When Xis a —OR group and R⁶ is a hydroxyl-protecting group, a plurality ofskeletons (N-oxy cyclic imide skeletons) derived from the N-substitutedcyclic imide skeleton by removal of R may be combined through R.

The hydroxyl-protecting group R in Formula (I) includes conventionalhydroxyl-protecting groups in the field of organic synthesis. Suchprotecting groups include, but are not limited to, alkyl groups (e.g.,methyl, t-butyl, and other C₁-C₄ alkyl groups), alkenyl groups (e.g.,allyl group), cycloalkyl groups (e.g., cyclohexyl group), aryl groups(e.g., 2,4-dinitrophenyl group), aralkyl groups (e.g., benzyl,2,6-dichlorobenzyl, 3-bromobenzyl, 2-nitrobenzyl, and triphenylmethylgroups); substituted methyl groups (e.g., methoxymethyl,methylthiomethyl, benzyloxymethyl, t-butoxymethyl,2-methoxyethoxymethyl, 2,2,2-trichloroethoxymethyl,bis(2-chloroethoxy)methyl, and 2-(trimethylsilyl)ethoxymethyl groups),substituted ethyl groups (e.g., 1-ethoxyethyl, 1-methyl-1-methoxyethyl,1-isopropoxyethyl, 2,2,2-trichloroethyl, and 2-methoxyethyl groups),tetrahydropyranyl group, tetrahydrofuranyl group, 1-hydroxyalkyl groups(e.g., 1-hydroxyethyl, 1-hydroxyhexyl, 1-hydroxydecyl,1-hydroxyhexadecyl, 1-hydroxy-1-phenylmethyl groups), and other groupsthat can form an acetal or hemiacetal group with a hydroxyl group; acylgroups (e.g., formyl, acetyl, propionyl, butyryl, isobutyryl, valeryl,pivaloyl, hexanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, lauroyl,myristoyl, palmitoyl, stearoyl, and other aliphatic C₁-C₂₀ acyl groups,and other aliphatic unsaturated or saturated acyl groups; acetoacetylgroup; cyclopentanecarbonyl, cyclohexanecarbonyl, othercycloalkanecarbonyl groups, and other alicyclic acyl groups; benzoyl,naphthoyl, and other aromatic acyl groups), sulfonyl groups (e.g.,methanesulfonyl, ethanesulfonyl, trifluoromethanesulfonyl,benzenesulfonyl, p-toluenesulfonyl, and naphthalenesulfonyl groups),alkoxycarbonyl groups (e.g., methoxycarbonyl, ethoxycarbonyl,t-butoxycarbonyl, and other C₁-C₄ alkoxy-carbonyl groups),aralkyloxycarbonyl groups (e.g., benzyloxycarbonyl, andp-methoxybenzyloxycarbonyl groups), substituted or unsubstitutedcarbamoyl groups (e.g., carbamoyl, methylcarbamoyl, and phenylcarbamoylgroups), groups derived from inorganic acids (e.g., sulfuric acid,nitric acid, phosphoric acid, and boric acid) by removal of OH group,dialkylphosphinothioyl groups (e.g., dimethylphosphinothioyl group),diarylphosphinothioyl groups (e.g., diphenylphosphinothioyl group), andsubstituted silyl groups (e.g., trimethylsilyl, t-butyldimethylsilyl,tribenzylsilyl, and triphenylsilyl groups).

When X is a —OR group, a plurality of skeletons (N-oxy cyclic imideskeletons) derived from the N-substituted cyclic imide skeleton byremoval of R may be combined through R. In this case, R includes, forexample, oxalyl, malonyl, succinyl, glutaryl, adipoyl, phthaloyl,isophthaloyl, terephthaloyl, and other polycarboxylic acyl groups;carbonyl group; methylene, ethylidene, isopropylidene, cyclopentylidene,cyclohexylidene, benzylidene, and other polyvalenthydro carbon groups,of which groups that can form an acetal bond with two hydroxyl groupsare preferred.

Preferred examples of R are hydrogen atom; groups that can form anacetal or hemiacetal group (bond) with a hydroxyl group; acyl groups,sulfonyl groups, alkoxycarbonyl groups, carbamoyl groups, and othergroups derived from acids (e.g., carboxylic acids, sulfonic acids,carbonic acid, carbamic acid, sulfuric acid, phosphoric acids, and boricacids) by removal of OH group, and other hydrolyzable protecting groupsthat can be eliminated by hydrolysis.

In Formula (I), n is 0 or 1. Specifically, Formula (I) represents afive-membered N-substituted cyclic imide skeleton when n is 0 andrepresents a six-membered N-substituted cyclic imide skeleton when n is1.

Typical examples of the imide compounds are imide compounds representedby Formula (1). In the substituents R¹, R², R³, R⁴, R⁵, and R⁶ in theimide compounds of Formula (1), the halogen atoms include iodine,bromine, chlorine, and fluorine atoms. The alkyl groups include, forexample, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, s-butyl,t-butyl, hexyl, decyl, dodecyl, tetradecyl, hexadecyl, and otherstraight- or branched-chain alkyl groups each containing from about 1 toabout 30 carbon atoms, of which those each containing from about 1 toabout 20 carbon atoms are preferred.

The aryl groups include, for example, phenyl and naphthyl groups. Thecycloalkyl groups include, for example, cyclopentyl and cyclohexylgroups. The alkoxy groups include, for example, methoxy, ethoxy,isopropoxy, butoxy, t-butoxy, hexyloxy, octyloxy, decyloxy, dodecyloxy,tetradecyloxy, octadecyloxy, and other alkoxy groups each containingfrom about 1 to about 30carbon atoms, of which alkoxy group searchcontaining from about 1 to about 20 carbon atoms are preferred.

The substituted oxycarbonyl groups include, but are not limited to,methoxycarbonyl, ethoxycarbonyl, isopropoxycarbonyl, butoxycarbonyl,t-butoxycarbonyl, hexyloxycarbonyl, decyloxycarbonyl,hexadecyloxycarbonyl, and other C₁-C₃₀ alkoxy-carbonyl groups, of whichC₁-C₂₀ alkoxy-carbonyl groups are preferred; cyclopentyloxycarbonyl,cyclohexyloxycarbonyl, and other cycloalkyloxycarbonyl groups, of whichcycloalkyloxycarbonyl groups each having 3 to 20 members are preferred;phenyloxycarbonyl, naphthyloxycarbonyl, and other aryloxycarbonylgroups, of which C₆-C₂₀ aryloxy-carbonyl groups are preferred;benzyloxycarbonyl, and other aralkyloxycarbonyl groups, of which C₇-C₂₁aralkyloxy-carbonyl groups are preferred.

The acyl groups include, but are not limited to, formyl, acetyl,propionyl, butyryl, isobutyryl, valeryl, pivaloyl, hexanoyl, octanoyl,decanoyl, lauroyl, myristoyl, palmitoyl, stearoyl, and other aliphaticC₁-C₃₀ acyl groups, of which aliphatic C₁-C₂₀ acyl groups are preferred,and other unsaturated or saturated aliphatic acyl groups; acetoacetylgroup; cyclopentanecarbonyl, cyclohexanecarbonyl, and othercycloalkanecarbonyl, and other alicyclicacyl groups; benzoyl, naphthoyl,and other aromatic acyl groups.

The acyloxy groups include, but are not limited to, formyloxy,acetyloxy, propionyloxy, butyryloxy, isobutyryloxy, valeryloxy,pivaloyloxy, hexanoyloxy, octanoyloxy, decanoyloxy, lauroyloxy,myristoyloxy, palmitoyloxy, stearoyloxy, and other aliphatic C₁-C₃₀acyloxy groups, of which C₁-C₂₀ acyloxy groups are preferred, and otherunsaturated or saturated aliphatic acyloxy groups; acetoacetyloxy group;cyclopentanecarbonyloxy, cyclohexanecarbonyloxy, and othercycloalkanecarbonyloxy, and other alicyclic acyloxy groups; benzoyloxy,naphthoyloxy, and other aromatic acyloxy groups.

The substituents R¹, R², R³, R⁴, R⁵, and R⁶ may be the same with ordifferent from one another. At least two of R¹, R², R³, R⁴, R⁵, and R⁶in Formula (1) may be combined to form a double bond, an aromatic ring,or a non-aromatic ring. The aromatic or non-aromatic ring has preferablyfrom about 5 to about 12 members and more preferably from about 6 toabout 10 members. The ring may be a heterocyclic ring or condensedheterocyclic ring, but it is often a hydrocarbon ring. Such ringsinclude, for example, non-aromatic alicyclic rings (e.g., cyclohexanering and other cycloalkane rings which may have a substituent,cyclohexene ring and other cycloalkene rings which may have asubstituent), non-aromatic bridged rings (e.g., 5-norbornene ring andother bridged hydrocarbon rings which may have a substituent), benzenering, naphthalene ring, and other aromatic rings (including condensedrings) which may have a substituent. The ring often comprises anaromatic ring. The ring may have a substituent. Such substituentsinclude, but are not limited to, alkyl groups, haloalkyl groups,hydroxyl group, alkoxy groups, carboxyl group, substituted oxycarbonylgroups, acyl groups, acyloxygroups, nitro group, cyano group, aminogroup, and halogen atoms.

One or more of the N-substituted cyclic imido group indicated in Formula(1) may be further formed on at least one of R¹, R², R³, R⁴, R⁵, and R⁶and/or on the double bond, aromatic ring, or non-aromatic ring formed bythe at least two of R¹, R², R³, R⁴, R⁵, and R⁶ . For example, when atleast one of R¹, R², R³, R⁴, R⁵, and R⁶ is an alkyl group containing twoor more carbon atoms, the N-substituted cyclic imido group maybe formedwith the adjacent two carbon atoms constituting the alkyl group.Likewise, when at least two of R¹, R², R³, R⁴, R⁵, and R⁶ are combinedto form a double bond, the N-substituted cyclic imido group may beformed with the double bond. When at least two of R¹, R², R³, R⁴, R⁵,and R⁶ are combined to form an aromatic or non-aromatic ring, theN-substituted cyclic imido group may be formed with the adjacent twocarbon atoms constituting the ring.

Preferred imide compounds include compounds represented by followingformulae:

wherein R¹¹, R¹², R¹³, R¹⁴, R¹⁵, and R¹⁶ are the same or different andare each one of hydrogen atom, halogen atoms, alkyl groups, aryl groups,cycloalkyl groups, hydroxyl group, alkoxy groups, carboxyl group,substituted oxycarbonyl groups, acyl groups, and acyloxy groups;

-   -   R¹⁷, R¹⁸, R¹⁹, R²⁰, R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are the        same or different and are each one of hydrogen atom, alkyl        groups, haloalkyl groups, hydroxyl group, alkoxygroups,        carboxylgroup, substituted oxycarbonyl groups, acyl groups,        acyloxy groups, nitro group, cyano group, amino group, and        halogen atoms,    -   wherein adjacent two of R¹⁷ to R²⁶ may be combined to forma        five- or six-membered N-substituted cyclic imide skeleton        indicated in one of Formulae (1c), (1d), (1e), (1f), (1h), and        (1i); and    -   X has the same meaning as defined above.

The halogen atoms, alkyl groups, aryl groups, cycloalkyl groups,hydroxyl group, alkoxy groups, carboxyl group, substituted oxycarbonylgroups, acyl groups, and acyloxy groups in the substituents R¹¹ to R¹⁶include the same groups as in the corresponding groups in thesubstituents R¹ to R⁶.

In the substituents R¹⁷ to R²⁶, the alkyl groups include the same alkylgroups as those exemplified above, of which alkyl groups each containingfrom about 1 to about 6 carbon atoms are preferred. The haloalkyl groupsinclude, for example, trifluoromethyl group, and other haloalkyl groupseach containing from about 1 to about 4 carbon atoms. The alkoxy groupsinclude the same alkoxy groups as those exemplified above, of whichlower alkoxy groups each containing from about 1 to about 4 carbon atomsare preferred. The substituted oxycarbonyl groups include the samesubstituted oxycarbonyl groups as those exemplified above, such asalkoxy carbonyl groups, cycloalkyloxycarbonyl groups, aryloxycarbonylgroups, and aralkyloxycarbonyl groups. The acyl groups include aliphaticunsaturated or saturated acyl groups, acetoacetyl group, alicyclic acylgroups, aromatic acyl groups, and other acyl groups as exemplifiedabove. The acyloxy groups include aliphatic unsaturated or saturatedacyloxy groups, acetoacetyloxy group, alicyclic acyloxy groups, aromaticacyloxy groups, and other acyloxy groups as exemplified above. Thehalogen atoms include, for example, fluorine, chlorine, and bromineatoms. Each of the substituents R¹⁷ to R²⁶ is often one of hydrogenatom, lower alkyl groups each containing from about 1 to about 4 carbonatoms, carboxyl group, substituted oxycarbonyl groups, nitro group, andhalogen atoms.

Examples of preferred imide compounds having a five-memberedN-substituted cyclic imide skeleton are N-hydroxysuccinimide,N-hydroxy-α-methylsuccinimide, N-hydroxy-α,α-dimethylsuccinimide,N-hydroxy-α,β-dimethylsuccinimide,N-hydroxy-α,α,β,β-tetramethylsuccinimide, N-hydroxymaleimide,N-hydroxyhexahydrophthalimide, N,N′-dihydroxycyclohexanetetracarboxylicdiimide, N-hydroxyphthalimide, N-hydroxytetrabromophthalimide,N-hydroxytetrachlorophthalimide, N-hydroxychlorendimide,N-hydroxyhimimide, N-hydroxytrimellitimide, N,N′-dihydroxypyromelliticdiimide, N,N′-dihydroxynaphthalenetetracarboxylic diimide,α,β-diacetoxy-N-hydroxysuccinimide,N-hydroxy-α,β-bis(propionyloxy)succinimide,N-hydroxy-α,β-bis(valeryloxy)succinimide,N-hydroxy-α,β-bis(lauroyloxy)succinimide,α,β-bis(benzoyloxy)-N-hydroxysuccinimide,N-hydroxy-4-methoxycarbonylphthalimide,4-ethoxycarbonyl-N-hydroxyphthalimide,N-hydroxy-4-pentyloxycarbonylphthalimide,4-dodecyloxy-N-hydroxycarbonylphthalimide,N-hydroxy-4-phenoxycarbonylphthalimide,N-hydroxy-4,5-bis(methoxycarbonyl)phthalimide,4,5-bis(ethoxycarbonyl)-N-hydroxyphthalimide,N-hydroxy-4,5-bis(pentyloxycarbonyl)phthalimide,4,5-bis(dodecyloxycarbonyl)-N-hydroxyphthalimide,N-hydroxy-4,5-bis(phenoxycarbonyl)phthalimide, and other compounds ofFormula (1) wherein X is a —OR group and R is a hydrogen atom; compoundscorresponding to these compounds except with R of an acyl group such asacetyl group, propionyl group, and benzoyl group;N-methoxymethyloxyphthalimide, N-(2-methoxyethoxymethyloxy)phthalimide,N-tetrahydropyranyloxyphthalimide, and other compounds of Formula (1)wherein X is a —OR group and R is a group that can form an acetal orhemiacetal bond with a hydroxyl group; N-methanesulfonyloxyphthalimide,N-(p-toluenesulfonyloxy)phthalimide, and other compounds of Formula (1)wherein X is a —OR group and R is a sulfonyl group; sulfuric esters,nitric esters, phosphoric esters, and boric esters ofN-hydroxyphthalimide, and other compounds of Formula (1) wherein X is a—OR group and R is a group derived from an inorganic acid by removal ofOH group.

Examples of preferred imide compounds having a six-memberedN-substituted cyclic imide skeleton are N-hydroxyglutarimide,N-hydroxy-α,α-dimethylglutarimide, N-hydroxy-β,β-dimethylglutarimide,N-hydroxy-1,8-decalindicarboximide,N,N′-dihydroxy-1,8;4,5-decalintetracarboxylic diimide,N-hydroxy-1,8-naphthalenedicarboximide (N-hyrdoxynaphthalimide),N,N′-dihydroxy-1,8;4,5-naphthalenetetracarboxylic diimide, and othercompounds of Formula (1) wherein X is a —OR group and R⁶ is a hydrogenatom; compounds corresponding to these compounds except with R of anacyl group such as acetyl group, propionyl group, and benzoyl group;N-methoxymethyloxy-1,8-naphthalenedicarboximide,N,N′-bis(methoxymethyloxy)-1,8;4,5-naphthalenetetracarboxy lic diimide,and other compounds of Formula (1) wherein X is a —OR group and R is agroup that can form an acetal or hemiacetal bond with a hydroxyl group;N-methanesulfonyloxy-1,8-naphthalenedicarboximide,N,N′-bis(methanesulfonyloxy)-1,8;4,5-naphthalenetetracarbo xylicdiimide, and other compounds of Formula (1) wherein X is a —OR group andR is a sulfonyl group; sulfuric esters, nitric esters, phosphoricesters, and boric esters of N-hydroxy-1,8-naphthalenedicarboximide andN,N′-dihydroxy-1,8;4,5-naphthalenetetracarboxylic diimide, and othercompounds of Formula (1) wherein X is a —OR group and R is a groupderived from an inorganic acid by removal of OH group.

Among the imide compounds, compounds wherein X is a —OR group and R isahydrogen atom (N-hydroxy cyclic imide compounds) can be prepared by aconventional imidization process such as a process that comprises thesteps of allowing a corresponding acid anhydride to react withhydroxylamine for ring-opening of an acid anhydride group, and closingthe ring to form an imide. Compounds of formula (1) wherein X is a —ORgroup and R is a hydroxyl-protecting group can be prepared byintroducing a desiredprotecting group into a corresponding compoundwherein R is a hydrogen atom (N-hydroxy cyclic imide compounds) by theaid of a conventional reaction for the introduction of protectinggroups. For example, N-acetoxy phthalimide can be prepared by allowingN-hydroxyphthalimide to react with acetic anhydride or to react with anacetyl halide in the presence of a base. These compounds can also beprepared by other processes.

Typically preferred imide compounds are N-hydroxysuccinimide,N-hydroxyphthalimide, N,N′-dihydroxypyromellitic diimide,N-hydroxyglutarimide, N-hydroxy-1,8-naphthalenedicarboximide,N,N′-dihydroxy-1,8;4,5-naphthalenetetracarboxylic diimide, and otherN-hydroxyimide compounds derived from alicyclic polycarboxylicanhydrides or aromatic polycarboxylic anhydrides; and compounds derivedfrom the N-hydroxyimide compounds by introduction of a protecting groupinto hydroxyl groups thereof.

Each of the imide compounds having at least one N-substituted cyclicimide skeleton represented by Formula (I) can be used alone or incombination in the reaction. The imide compounds can be formed in areaction system.

The amount of the imide compound(s) can be selected within broad rangesand is, for example, from about 0.0000001 to about 1 mole, preferablyfrom about 0.000001 to about 0.5 mole, more preferably from about0.00001 to about 0.4 mole, and often from about 0.0001 to about 0.35mole, per mole of the cycloalkane (substrate). The amount of the imidecompound(s) is, for example, from about 0.0000006 to about 6 moles, andpreferably from about 0.0006 to about 2.1 moles per kg of the totalcharged liquid.

Promoters (Co-Catalysts)

A promoter (a co-catalyst) can be used in the reaction. The combinationuse of the catalyst with the promoter can improve or enhance the rateand/or selectivity of the reaction. Such promoters include, for example,organic salts comprising a polyatomic cation or a polyatomic anion andits counter ion, which polyatomic cation or anion contains a Group 15 orGroup 16 element of the Periodic Table of Elements having at least oneorganic group combined therewith.

In the organic salts, the Group 15 elements of the Periodic Table ofElements include N, P, As, Sb, and Bi, and the Group 16 elements of thePeriodic Table of Elements include, for example, O, S, Se and Te.Preferred elements are N, P, As, Sb, and S, of which N, P, and S aretypically preferred.

The organic groups to be combined with the atoms of elements include,but are not limited to, aliphatic hydrocarbon groups, alicyclichydrocarbon groups, aromatic hydrocarbon groups, and other hydrocarbongroups which may have a substituent; and alkoxy groups, aryloxy groups,aralkyloxy groups, and other substituted oxy groups.

Examples of the organic salts are organic ammonium salts, organicphosphonium salts, organic sulfonium salts, and other organic oniumsalts. Examples of organic ammonium salts include tetramethylammoniumchloride, tetrabutylammonium chloride, triethylphenylammonium chloride,other quaternary ammonium chlorides, corresponding quaternary ammoniumbromides, and other quaternary ammonium salts each having fourhydrocarbon groups combined with its nitrogen atom; dimethylpiperidiniumchloride, hexadecylpyridinium chloride, methylquinolinium chloride, andother cyclic quaternary ammonium salts. Examples of the organicphosphonium salts include tetramethylphosphonium chloride,tetrabutylphosphonium chloride, other quaternary phosphonium chlorides,corresponding quaternary phosphonium bromides, and other quaternaryphosphonium salts each having four hydrocarbon groups combined with itsphosphorus atom. Examples of the organic sulfonium salts includetriethylsulfonium iodide, ethyldiphenylsulfonium iodide, and othersulfonium salts each having three hydrocarbon groups combined with itssulfur atom.

The organic salts also include methanesulfonates, dodecanesulfonates,and other alkyl-sulfonates (e.g., C₁-C₁₈ alkyl-sufonates);benzenesulfonates, p-toluenesulfonates, naphthalenesulfonates, and otheraryl-sulfonates which may be substituted with an alkyl group (e.g.,C₁-C₁₈ alkyl-arylsufonates); sulfonic acid type ion exchange resins (ionexchangers); and phosphonic acid type ion exchange resins (ionexchangers).

The amount of the organic salt(s) is, for example, from about 0.001 toabout 0.1 mole, and preferably from about 0.005 to about 0.08 mole permole of the catalyst.

Strong acids such as compounds having a pKa of less than or equal to 2(at 25° C.) can also be used as the promoter. Preferred examples ofstrong acids are hydrogen halides, hydrohalogenic acids, sulfuric acid,and heteropolyacids. The amount of the strong acid(s) is, for example,from about 0.001 to about 3 moles per mole of the catalyst.

The promoters for use in the present invention also include compoundshaving a carbonyl group combined with an electron attractive group.Examples of such compounds are hexafluoroacetone, trifluoroacetic acid,pentafluorophenyl (methyl) ketone, pentafluorophenyl (trifluoromethyl)ketone, and benzoic acid. The amount of the compound(s) is, for example,from about 0.0001 to about 3 moles per mole of the cycloalkane(substrate).

The reaction system may further comprise a free-radical generator or afree-radical reaction accelerator. Such components include, but are notlimited to, halogens (e.g., chlorine and bromine), peracids (e.g.,peracetic acid and m-chloroperbenzoic acid), and peroxides (e.g.,hydrogen peroxide, t-butyl hydroperoxide (TBHP), and otherhydroperoxides), nitric acid, nitrous acid, and salts thereof, nitrogendioxide, benzaldehyde, and other aldehydes. The existence of thecomponent(s) in the system may enhance a reaction. The amount of theaforementioned component(s) is, for example, from about 0.001 to about 3mole per mole of the catalyst.

Reactions

The reaction is performed in a liquid phase using a continuous reactor.A reaction vessel in the reactor can be of any type such as a completemixing vessel and a plug flow vessel.

Reaction solvents for use in the present invention include, but are notlimited to, benzene and other aromatic hydrocarbons; dichloromethane,chloroform, 1,2-dichloroethane, dichlorobenzene, and other halogenatedhydrocarbons; t-butyl alcohol, t-amyl alcohol, and other alcohols;acetonitrile, benzonitrile, and other nitriles; acetic acid, propionicacid, and other carboxylic acids; formamide, acetamide,dimethylformamide (DMF), dimethylacetamide, and other amides. Each ofthese solvents can be used alone or in combination. The reaction productdicarboxylic acid can also be used as the reaction solvent. Among thesolvents, carboxylic acids and other organic protonic solvents as wellas nitriles are preferred, of which acetic acid, and other carboxylicacids are typically preferred. The reaction may be performed withoutsupply of the reaction solvent.

One of the features of the present invention is that a residence time τ(hr) in the continuous reactor is set so as to satisfy the followingcondition:0.1≦τ≦50/c

wherein c is the proportion (% by weight) of the cycloalkane based onthe total weight of the charged liquid (the total of the cycloalkane,solvent, catalyst, and other components). The residence time τ (hr) canbe determined by calculation according to the following equation.τ (hr)=[Amount of liquid in the reaction vessel (L)]/[Flow rate of thecharged liquid (L/hr)]

If the residence time is shorter than 0.1 hour, the conversion from thecycloalkane decreases. With a gradually increasing residence time from0.1 hour, the conversion from the cycloalkane gradually increases.However, from a certain midpoint, other dicarboxylic acids than thetarget dicarboxylic acid (a dicarboxylic acid with a carbon chaincontaining carbon atoms in the same number as the carbon atomsconstituting the cycloalkane ring), i.e., dicarboxylic acids each with acarbon chain containing carbon atom(s) in a number one or more less thanthat of the carbon atoms constituting the cycloalkane ring, increase,and the reactivity decreases. As a result, the space-time yield (STY) ofthe target dicarboxylic acid significantly decreases.

For example, when the residence time is increased in the production ofadipic acid by oxidation of cyclohexane using a continuous reactor, thereactivity decreases at or above some middle point, and the selectivityof adipic acid on the basis of total dicarboxylic acids (the total ofadipic acid, glutaric acid, and succinic acid) gradually decreases. Withan increasing residence time, by-products such as hydroxycaproic acid,butyrolactone, and valerolactone increase, but at or above somemidpoint, these by-produced compounds decrease. These results suggestthat the by-products convert to other substances in such a long-timereaction, and some of the converted substances may adversely affect theactivity of the catalyst such as a cobalt compound or manganesecompound. Japanese Unexamined Patent Application Publication No. 50-8790describes that when cyclohexane is oxidized with oxygen in a batchsystem and a cobalt compound catalyst is reused over again, thecatalytic activity gradually decreases, and the catalyst having thedecreased activity is activated by treatment with an organic solvent.These suggest that the inhibitor is an organic compound and poisons thecatalytic metal to thereby deteriorate the catalytic activity by, forexample, the formation of a metallic complex.

An optimum residence time varies depending on the ratio of thecycloalkane to the total charged liquid supplied to the reactionvessel(concentration of the cycloalkane in the charged liquid). If theconcentration of the cycloalkane in the charged liquid is high, acycloalkane phase separates from an aqueous phase in the reactionsystemwith a slightly increasing residence time due to water formedduring the reaction, and the reactivity rapidly decreases. The residencetime immediately before the rapid decrease of the reactivity is anoptimum residence time in this case. If the cycloalkane concentration inthe charged liquid is low and the cycloalkane phase does not separatefrom the aqueous phase in the reaction system, the reactivity decreasesdue to the reaction inhibitor with an increasing residence time. In thiscase, the residence time immediately before the decrease of theselectivity of the target dicarboxylic acid on the basis of the totaldicarboxylic acids is an optimum residence time. These findings showthat the upper limit of a preferred residence time is substantiallyinversely proportional to the cycloalkane concentration in the chargedliquid and is expressed by 50/c, wherein c has the same meaning asdefined above.

The lower and upper limits of the residence time are preferably 0.2 hourand 40/c, wherein c has the same meaning as defined above, respectively.The concentration c is preferably equal to or more than 15% by weight(e.g., from 15% to 99.5% by weight), more preferably equal to or morethan 18% by weight (e.g., from 18% to 95% by weight), more preferablyequal to or more than 20% by weight (e.g., from 20% to 80% by weight),and typically preferably equal to or more than 25% by weight (e.g., from25% to 60% by weight). If the concentration c is excessively low, theconversion speed of the cycloalkane may become excessively low, and thespace-time yield (yield per unit volume and per unit time) of theproduced dicarboxylic acid may decrease.

The present inventors have found that, when a cobalt compound is used asthe catalyst, the ratio of Co(II) to Co(III) in the reaction mixture issubstantially invariant of about 90:10 regardless of the residence time.It is believed that Co(III) exhibits catalytic activity and Co(II) doesnot in oxidation reactions of cycloalkanes (e.g., Kogyo Kagaku Zasshi(Journal of the Chemical Society of Japan, Industrial chemistrysection), 72(12), 2590 (1969)). The fact that the ratio of Co(II) toCo(III) in the reaction system is invariant indicates that cobalt ispoisoned without changing its oxidation number and decreases in itsreaction activity by, for example, forming a complex with the reactioninhibitor.

A reaction temperature is, for example, from 80° C. to 200° C.,preferably from 80° C. to 150° C., and more preferably from 90° C. to140° C.. If the reaction temperature is lower than 80° C., the reactionrate (reaction speed) may decrease. If it is excessively high, theselectivity of the target dicarboxylic acid may often decrease. Thereaction can be performed at atmospheric pressure or under a pressure(under a load). When the reaction is performed under a pressure, thereaction pressure is, for example, equal to or more than about 0.5 MPa(e.g., from about 0.5 to about 20 MPa), and preferably from about 1 toabout 15 MPa.

As a result of the reaction, the material cycloalkane oxidativelycleaves and thereby mainly yields a dicarboxylic acid with a carbonchain containing carbon atoms in the same number as carbon atomsconstituting the cycloalkane ring. Specifically, cyclohexane yieldsadipic acid, and cyclododecane yields a dodecanedicarboxylic acid. Undersome conditions, dicarboxylic acids with a carbon chain containingcarbon atoms in a number one or two less than the number of carbon atomsconstituting the cycloalkane ring, and/or corresponding cycloalkanolsand cyckoalkanones may be by-produced. For example, glutaric acid,succinic acid, cyclohexanol, cyclohexanone, acetic acid, cyclohexylacetate, lactones such as butyrolactone and valerolactone, adipicesters, and/or hydroxycaproic acid may be by-produced from materialcyclohexane. Among these by-products, cycloalkanols and cycloalkanonescan be recycled into the reaction system.

The reaction product(s) can be separated and purified by separationmeans such as filtration, concentration, distillation, extraction,crystallization, recrystallization, adsorption, column chromatography,and combinations thereof.

Dicarboxylic acids obtained by the production process of the presentinvention can be used as, for example, rawmaterials for polyamides(nylons) and polyesters, additives for polymers such as polyurethanes,and intermediate materials for fine chemicals.

EXAMPLES

The present invention will be illustrated in further detail withreference to several examples and comparative examples below, which arenot intended to limit the scope of the invention. Products in theexamples and comparative examples were analyzed by gas chromatographyand high-performance liquid chromatography. In the following tables, theabbreviations CHX, STY, and ADA mean cyclohexane, the space-time yield(kg-ADA/m³.hr) and adipic acid, respectively.

Examples 1 to 3 and Comparative Example 1

Cyclohexane (CHX), acetic acid (AcOH), and cobalt(II) acetate andN-acetoxy phthalimide as catalysts were continuously supplied to a1000-ml reactor made of titanium equipped with a three-stage puddleagitator (revolutions: 500 rpm) so that the residence time τ (hr) was aset value. The residence time was controlled by changing the supplyamount of the materials. The materials were supplied in two lines, aline for supplying cyclohexane and a line for supplying a solution ofthe catalysts inacetic acid. Thesematerial supply lines meet with eachother at an inlet of the reactor, and the materials were supplied fromthe top of the reactor through an insertion tube to a liquid phase.

The weight ratio of supplied cyclohexane CHX to supplied acetic acidAcOH is 30/70. The amounts of cobalt(II) acetate and N-acetoxyphthalimide were 21 mmol/kg and 23 mmol/kg, respectively, relative tothe total weight of the charged materials. The concentration c ofcyclohexane to the total charged materials was 30% by weight. Theresults of reactions are shown in Table 1. TABLE 1 Conversion ResidenceTime from CHX (hr) (%) STY Example 1 0.40 8.8 53.5 Example 2 0.85 15.851.5 Example 3 1.50 21.7 45.5 Comparative 2.01 23.0 30.9 Example 1

Table 1 shows that adipic acid is obtained in high space-time yieldsaccording to Examples 1 to 3 in which the residence time τ satisfies thefollowing condition: 0.1≦τ≦1.7 (=50/c) but is obtained in a markedly lowspace-time yield according to Comparative Example 1 in which theresidence time τ does not satisfy the above condition.

Examples 4 to 6 and Comparative Example 2

Cyclohexane (CHX), acetic acid (AcOH), and cobalt(II) acetate as acatalyst were continuously supplied to a 1000-ml reactor made oftitanium equipped with a three-stage puddle agitator (revolutions: 500rpm) so that the residence time τ (hr) was a set value. The residencetime was controlled by changing the supply amount of the materials. Thematerials were supplied in the same manner as in Examples 1 to 3 andComparative Example 1.

The weight ratio of supplied cyclohexane CHX to supplied acetic acidAcOH is 30/70. The amount of cobalt(II) acetate was 21 mmol/kg relativeto the total weight of-the charged materials. The concentration c ofcyclohexane to the total charged materials was 30% by weight. Theresults of reactions are shown in Table 2. TABLE 2 Conversion ResidenceTime from CHX (hr) (%) STY Example 4 0.37 7.7 60.1 Example 5 0.72 14.257.9 Example 6 1.40 18.8 48.3 Comparative 1.91 19.7 29.0 Example 2

Table 2 shows that adipic acid is obtained in high space-time yieldsaccording to Examples 4 to 6 in which the residence time τ satisfies thefollowing condition: 0.1≦τ≦1.7 (=50/c) but is obtained in a markedly lowspace-time yield according to Comparative Example 2 in which theresidence time τ does not satisfy the above condition.

Example 7 and Comparative Example 3

The procedures of Examples 1 to 3 and Comparative Example 1 wererepeated, except that the weight ratio of supplied cyclohexane tosupplied acetic acid was changed to 60/40. The concentration c ofcyclohexane to the total charged materials was 60% by weight. Theresults of reactions are shown in Table 3. TABLE 3 Residence ConversionTime from CHX (hr) (%) STY Example 7 0.72 9.9 65.0 Comparative 1.77 9.028.0 Example 3

Table 3 shows that adipic acid is obtained in a high space-time yieldaccording to Examples 7 in which the residence time τ satisfies thefollowing condition: 0.1≦τ≦0.83 (=50/c) but is obtained in a markedlylow space-time yield according to Comparative Example 3 in which theresidence time τ does not satisfy the above condition.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

1. A process for producing a dicarboxylic acid comprising the step ofsubjecting a cycloalkane to an oxidative cleavage reaction with oxygenin the presence of a catalyst in a liquid phase using a continuousreactor to thereby yield a corresponding dicarboxylic acid, wherein aresidence time τ (hr) satisfies the following condition:0.1≦τ≦50/c wherein c is the proportion (% by weight) of the cycloalkaneto the total weight of a charged liquid.
 2. The process according toclaim 1, wherein the catalyst comprises one selected from cobaltcompounds, manganese compounds, and mixtures of these compounds.
 3. Theprocess according to claim 1, wherein the catalyst comprises an imidecompound having at least one cyclic imide skeleton represented byfollowing Formula (I):

wherein n is 0 or 1; and X is an oxygen atom or a —OR group, wherein Ris a hydrogen atom or a hydroxyl-protecting group.
 4. The processaccording to claim 3, wherein the imide compound is a compoundrepresented by following Formula (1):

wherein n is 0 or 1; X is an oxygen atom or a —OR group wherein R is ahydrogen atom or a hydroxyl-protecting group; R¹, R², R³, R⁴, R⁵ and R⁶are the same or different and are each one selected from the groupconsisting of hydrogen atom, halogen atoms, alkyl groups, aryl groups,cycloalkyl groups, hydroxyl group, alkoxy groups, carboxyl group,substituted oxycarbonyl groups, acyl groups, and acyloxy groups, whereinat least two of R¹, R², R³, R⁴, R⁵ and R⁶ may be combined to form one ofa double bond, an aromatic ring and a non-aromatic ring, and wherein oneor more of the N-substituted cyclic imido group indicated in Formula (1)may be formed on at least one of R¹, R², R³, R⁴, R⁵ and R⁶ and/or on thedouble bond, aromatic ring or non-aromatic ring formed by the at leasttwo of R¹, R², R³, R⁴, R⁵ and R⁶.
 5. The process according to claim 1,further comprising performing the oxidative cleavage reaction in thepresence of a carboxylic acid as a reaction solvent.
 6. The processaccording to claim 1, further comprising performing the oxidativecleavage reaction at a reaction temperature of 80° C. to 150° C.
 7. Theprocess according to claim 1, further comprising performing theoxidative cleavage reaction at a reaction pressure of equal to or morethan 0.5 MPa.